This invention relates to microencapsulation. More particularly, it relates to a method of loading permeable microcapsules and a composition of matter comprising a permeable microcapsule containing one or more semipermeable capsules containing the material substances which may be released from the intracapsular volume at a substantially constant rate for a significant time.
Compositions of matter capable of sustained release of drugs, fertilizers, and the like are well known in the art. Generally, such compositions comprise a solid carrier in which the substance to be released is adsorbed or trapped as plural separate phases. When such compositions are placed in the environment in which they are intended for use, the outer layers of the solid carrier dissolve releasing a portion of the substance of interest. If the solid carrier dissolves slowly, then the substance of interest is released slowly or in discrete bursts over time. In use, the rate of release of the substance from such compositions necessarily is dependent upon the solubility of the carrier particle material, on the surface area of the carrier particle that at any given time is exposed to the environment, and often on the solubility of the substance in the environment of use. These factors typically result in a release rate which decreases over time.
In situations where it is desired, e.g., to release a drug at a constant rate into the circulatory system, the foregoing approach based on solubility properties cannot be used. In such situations, implantable mechanical perfusion pumps and similar devices have been suggested.